I'd contact your department of health and human services. In order for me to cater in NH, I first had to have a commercial kitchen which licensing is handled through NHDHS. For the most part, the concern is what sort of kitchen you're working out of and I'm sure that varies from state to state. Once you have a commercial facility, I don't think it matters much what you do out of it. NH doesn't have a catering license exactly, but a food service license which would cover catering onsite or offsite.
Again, I can only speak for my state, but it wasn't all that hard to get a license for my kitchen. There are septic/sewage requirements, water tests, refrigeration requirements, floor/wall covering requirements. I had to hire a plumber to install about 100 sinks to the tune of about a million $$. Not really- just 5 sinks (it was supposed to be 6, but we're cooperating...)- 3 bay for washing, food prep for ...well...food prep, at least one hand sink depending upon the size of our kitchen, mop sink, hand sink in bathroom. It still cost me a ton of $$ because the plumber is a knucklehead- he tore out 2 walls to find the pipes he had installed 5 years earlier in only one of the 2 walls he ripped out and that's just the beginning.... I still have buckets under 2 of the 3 drains in my 3 bay. But enough about me..:beer: